Electromagnetic pulses can induce very high currents and voltages on cables and structures such as aeroplanes and ships, which can penetrate the electrical systems and damage them, either causing hardware damage such as semiconductor burnout, or electronic upset, e.g. transmission loss or loss or stored data. As semiconductor technology advances, the energy needed to cause such damage becomes less.
The most severe threat to electrical and electronic equipment is the electromagnetic pulse due to detonation of a nuclear device which can generate large voltage or current spikes with very short rise times. The exact characteristics of the pulse are complicated although a typical pulse will exhibit a field strength of about 10.sup.5 Vm.sup.-1 with an accompanying H field intensity of 130 Am.sup.-1 (based on free space impedance of 377 ohms), a pulse width of a few microseconds and a rise time of a few nanoseconds. In general damaging amplitudes in the pulse occur within the frequency range of 10 KHz to 100 MHz.
In many circumstances it may be necessary to protect equipment from voltage transients other than those due to nuclear detonation. For example, although lightning is not normally associated with transients having such short rise times, it has recently been considered that certain components in lightning transients may, in fact, exhibit very short rise times and so may not be removed by conventional surge arrestors.